The present invention relates to mechanical expansion anchors for use with elongated bolts and bearing plates in the reinforcement and support of rock formations such as mine roofs, and to methods of fabrication and installation thereof. More specifically, the invention relates to novel means for inhibiting rotation of a mechanical anchor within a drill hole as the bolt is rotated to effect expansion of the anchor shell.
Mechanical expansion anchors, sometimes used in combination with a hardenable resin to provide a combined mechanical-chemical anchor, have for many years been the most widely used means of reinforcing and supporting underground rock formations such as mine roofs. Conventional expansion anchors include a hollow shell structure having a plurality of leaves arranged about a central axis, and a tapered camming plug. The small end of the plug extends into one end of the shell and the leaves are moved radially outwardly from the central axis by movement of the progressively larger portion of the plug into the area between the leaves. The anchor is carried on one end of an elongated bolt which is inserted in a drill hole of predetermined depth and diameter in the rock structure.
The distal end of the bolt, i.e., the end of the bolt which is inserted into the hole is externally threaded, and a central, through opening in the tapered plug is internally threaded for mating engagement with the bolt. Means such as a bail or support nut are provided for maintaining the plug and shell in assembled relation on the bolt, and for restraining axial movement of the shell while rotation of the bolt moves the plug into the shell to effect expansion thereof. Support nuts used in such applications are often fabricated of sheet metal with only a single full thread revolution. Many types and configurations of bails have been devised, some of which are specifically discussed hereinafter.
In order for the shell to expand in the intended manner, it is necessary that the anchor be restrained against rotation as the bolt is rotated, and that the shell be restrained against axial movement as the plug moves axially upon the bolt due to its threaded engagement therewith. Frictional contact of the outer surface of the shell with the bore hole wall, and engagement of the small end of the plug in the end of the shell are usually relied upon to provide the required rotational constraint, although other means associated with the anchor have also been proposed. The support nut or bail, each of which are in mutual engagement with both the shell and the bolt, provide the restraint against axial movement of the shell.
A problem which sometimes arises in the use of mechanical mine roof anchors is that of so-called "spinners." That is, due to such factors as soft rock strata, bore hole diameter greater than that of the shell, binding or galling of the threaded connection between bolt and plug, etc., the anchor may not be rotationally restrained to the extent required. The entire anchor then rotates together with the bolt and does not expand. Obviously, in such cases no anchorage for the bolt and no support or reinforcement for the rock formation is provided.
One previously disclosed expansion anchor having means intended to prevent spinners is that of U.S. Pat. No. 5,033,910 of the present inventor wherein friction teeth extend integrally from the outer surface of the shell. These teeth are intended to extend outwardly a distance sufficient to positively engage the bore hole wall and ensure that the anchor does not spin as the bolt is rotated. Being a rigid portion of the shell, there is the possibility that such teeth may interfere with free insertion of the bolt and anchor into the bore hole. Conversely, if the bore hole is oversize, the rigid teeth may not provide sufficient frictional contact with the bore hole wall to provide the required rotational constraint.
Other means for engaging the bore hole wall to inhibit rotation of the anchor have been proposed as a part of or in association with bail structure. For example, U.S. Pat. No. 5,018,908 of Laphon discloses an anchor including a bail having arms extending outwardly from the upper, medial portion of the bail which is positioned over the tapered plug. The arms are resiliently deformable as the anchor is inserted in the bore hole, and terminal ends of the arms engage the bore hole wall to retard or inhibit rotation of the anchor. The arms are separate from and in addition to the portions of the bail structure which extend from the medial portion for engagement with the shell. Furthermore, as the arms engage the bore hole wall they are flexed inwardly against the upper end of the plug, thus causing an undesired resistance to downward movement of the plug to effect shell expansion.
While the advantages of means frictionally engaging the bore hole wall to inhibit rotation of the anchor during shell expansion have been recognized, the previously disclosed means have not proved entirely satisfactory. Accordingly, it is a primary object of the present invention to provide novel and effective means associated with a mine roof expansion anchor for frictionally engaging the bore hole wall in the unexpanded condition of the anchor to inhibit rotation thereof.
It is a further object to provide an expansion anchor having a tapered plug which is moved axially away from the distal end of a threaded bolt in response to bolt rotation to effect radial expansion of a shell within a bore hole including means for inhibiting rotation of the anchor as the bolt is rotated without interfering with axial movement of the plug to effect shell expansion.
Another object is to provide a bail-type mine roof expansion anchor having conventional expansible shell and tapered plug structure in combination with a bail having tabs or arms which extend outwardly from the unexpanded shell to frictionally engage the bore hole wall at a location which effectively inhibits rotation of the anchor while ensuring shell expansion in the intended manner.
A still further object is to provide a novel and improved bail configuration for use with an otherwise conventional mine roof expansion anchor.
Still another object is to provide a novel and improved method of installing a mine roof expansion anchor in a bore hole on the threaded, distal end of an elongated bolt to significantly enhance proper installation of the anchor and tensioning of the bolt.
Other objects will in part be obvious and will in part appear hereinafter.